It requires expensive equipment and a high level of skill to do board level repair. It's rarely worth it when a replacement board can be had for $50 - $250. Even if it is a really expensive $1,000+ motherboard, not all boards can be fixed, and even if they can technically be fixed, good luck finding the little $0.03 part. And even if you do find the $0.03 part, they usually require a minimum order of 10,000 units with a 6 month lead time.
In other words, this business just isn't viable. It's barely worth it for Louis Rossman, and he works on super expensive MacBooks. It only works for him because:
1. MacBooks are extremely expensive so people are more likely to approve a $500+ repair
2. There are only a few models he has to familiarize himself with
3. There are no shortages of stupid Apple users who keep buying these things
But even Louis Rossman is looking to get out of the business. If that guy can't make it work, no one can. He's a really good businessman who is highly skilled and has everything going for him (other than his location). I think he could make a lot more by moving to the middle of nowhere where the rent isn't $25,000+/month and having people ship stuff to him.
